The invention is directed to an improved method and apparatus for measuring the air intake mass to an internal combustion engine.
Experience with devices and methods of this type has shown that when used in motor vehicle for measuring the mass, or flow rate, of the air aspirated by the engine, an undesirably long period of time elapses between when the device is turned on and the time that it functions at an accuracy of, for instance, 10% with an idling aspirated air quantity of approximately 10 kg/h. The delay is partly due to the necessity for the temperature-dependent resistor to heat up to the specified resistance temperature. If there is a sudden change in the mass of aspirated air in the engine, such as when changing from the idling range of approximately 10 kg/h to the full load range of about 300 kg/h, or from full load at about 300 kg/h to idling at about 10 kg/h, then there is a delay, known as response time, before the desired accuracy is attained, that is, the device reacts to abrupt changes in the aspirated air mass only in a delayed manner.
These undesirably long startup or response times when the aspirated air mass is varying mean that if the output signal of the device is used for triggering a fuel injection system, the injected fuel quantity will not match the aspirated air quantity until after delay. Not only does this increase the fuel consumption, but it also means a higher proportion of toxic ingredients in the exhaust gas. Tests have shown that the unfavorable behavior of the device when it is turned on and when there are sudden relatively large changes in the aspirated air quantity is due to the fact that while the coarse adjustment of the temperature profile effected along the length of the substrate of the device as a result of the regulated supply of energy does happen very quickly, nevertheless, in order to effect an exact adjustment of a new temperature profile in the unheated peripheral area of the substrate, a specific quantity of heat is needed; this heat is taken from the heated portion of the substrate, in the form of a small flow of heat which is therefore of correspondingly long duration. Although the resistance temperature of the temperature-dependent resistor is regulated to a constant value regardless of the air mass or flow rate, still the temperature of the peripheral area does vary in accordance with the flow rate of the aspirated air; as a result, the temperature profile along the length of the substrate varies as the aspirated air flow rate varies. The flow of heat to the peripheral areas of the substrate becomes particularly important in percentage terms with small aspirated air masses, that is when there is little cooling capacity and hence a low electrical heating output with the result being an incorrect measurement.